Tuesday, 18 March 2014

Why we need a CNC pick-n-place

It was good to spend a few hours this weekend actually making stuff again, rather than just coding and coding and doing yet more coding! So good, that we thought it was time to make a few more board sections for our electronic board game.

In about two hours we managed to get just two pieces made up. Mostly this was down to dabbing the solder paste onto just three or four sets of pads at a time, then hand-soldering each SOT-23 sized hall effect sensor, and each 1206 sized resistor in place.

Hand-soldering SMT components is slow going. And almost an hour per board section is pretty woeful. So it's time to start seriously thinking about this CNC pick-n-place machine. Steve is going to go beserk.

I went down to BuildBrighton for a few hours this evening, with Grumpy Paul (he doesn't have a G+ link). Over a few hours, we cobbled together a basic x/y axis, using some 12mm linear bearings and stainless steel rods that have been lying around for ages. We've mounted the steppers onto the y-axis carriage - one on each side rather than a single long belt down the middle, as we need to keep the middle clear for placing components. Also, because we're expecting the head to travel to the edge(s) to pick up components from a tape reel, we couldn't really use the moving y-axis bed approach we implemented in our CNC drill from about 18 months ago.

We've gone for "proper" stepper motors this time, and a 2mm pitch timing belt, and proper CNC pulleys (rather than some cheap plastic mouldings and £2 motors from China!)

Here's the x-axis carriage being assembled. The steppers are mounted on each end. A belt will run along the outside edges, over the pulleys on the motors that are mounted on the carriage, to a fixed point at each end of the travel - using this sort of arrangement:

As usual, the entire thing is being designed and built on-the-fly.
There's no real planning gone into the design so far (as Grumpy Paul will testify, having had to take a rasp file to it on more than one occasion). We're using Arthur's method of getting everything square - that is, to fix one end, run the carriage up and down the y-axis a few times, then bolt it all down onto a scrap sheet of wood, wherever it rests!

The travel on the y-axis is surprisingly smooth. Maybe it's because of the added weight of the stepper motors (we expected these to make the travel worse, not better). Maybe it's because we lightly sanded the rods with some wet-n-dry paper, as the linear rails felt a bit sticky in places. But in all honesty, it's probably Paul's idea that made all the difference - a tiny blob of lithium-based grease on the rods makes the carriages glide up and down with hardly any resistance at all.

We didn't actually get much further than laying everything out tonight (despite nearly five hours, and staying 'til almost midnight). But there's every chance that we'll have the CNC moving in at least two axes on Thursday night. That's if Steve doesn't put his foot down and ban us from doing any more work on it 'til the board game is finished!

The thing is, at our current rate of construction, we're going to need about 20 more hours just to make up enough board sections to call it complete. Surely spending a few hours to build the tools to speed up finishing off the board game it time well spent? Isn't it?